Method for operating an automatic start/stop system in a vehicle utilizing a fluid launch clutch

ABSTRACT

A system and method for operating a vehicle equipped with an automatic stop and start system is disclosed. The vehicle includes an internal combustion engine, an automatic transmission and a fluid launch device with an impeller disconnect clutch. A controller may initiate an automatic stop or start of the engine under certain operating conditions. During an engine start/stop event, the engine is automatically shut down and the impeller clutch of the fluid launch device may be disengaged to decouple the engine and transmission from the driveline to provide for improved fuel economy and reduced emissions.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a divisional of U.S. application Ser. No.15/145,329, filed May 3, 2016, the disclosure of which is herebyincorporated in its entirety by reference herein.

TECHNICAL FIELD

This disclosure relates to a method and a system for operating anautomatic start/stop system in a motor vehicle having an internalcombustion engine, a transmission and a fluid launch device with animpeller disconnect clutch.

BACKGROUND

Many automotive vehicles are equipped with an automatic start/stopsystem, which automatically shuts down and restarts the engine of avehicle to reduce fuel consumption of the vehicle under certainconditions. In operation, the internal combustion engine can be stopped,i.e. switched off, automatically by the automatic start/stop system ifno propulsion is required, for example by idling at a traffic light, andcan be restarted if the driver calls for power again.

One type of start/stop system is known as a rolling start/stop. Arolling start/stop system involves stopping the internal combustionengine when the vehicle is moving and decelerating. When power from theengine is required, e.g., the driver releases the brakes, the engine isautomatically restarted. The engine may also be automatically started inresponse to other conditions such as battery state of charge or loads onthe electrical system. These engine auto stops may improve fuel economyand reduce emissions by reducing engine idle time and thus fuelconsumption for the drive cycle. However, a rolling start/stop in avehicle with an automatic transmission connected to the drivetrain via aconventional torque converter may result in objectionable noise,vibration, and harshness (NVH) or drivability. One strategy to mitigatethese effects is to automatically shift the transmission into neutral,although this requires rapid reengagement when power is demanded and mayresult in a shift bump that is also objectionable to some occupants.

SUMMARY

A system and method for operating an automatic start/stop system in amotor vehicle having an internal combustion engine, a transmission, anda fluid launch device include operating an impeller disconnect clutchduring a rolling auto stop event. Embodiments according to the presentdisclosure may be implemented in various applications to improve fueleconomy.

According to an embodiment of the present disclosure, a vehicle includesan engine, a transmission and a launch device. The launch device mayinclude a disconnect clutch for selectively coupling the engine and thetransmission. The vehicle may also include a controller configured to,in response to the engine being stopped when a brake pedal is depressedand a vehicle speed is above a first speed threshold associated withvehicle idle, disengage the disconnect clutch to decouple the engine andthe transmission. The controller may further be configured to disengagethe disconnect clutch in response to the engine being stopped when thevehicle speed is below a second speed threshold associated with arolling stop threshold limit.

In another embodiment, a start/stop system for a vehicle having anengine, a transmission and a launch device including a disconnect clutchis disclosed. The start/stop system may include a controller configuredto, in response to an engine being alternately stopped and restartedduring a rolling auto stop event, control the disconnect clutch of thelaunch device to selectively couple the engine and the transmission. Thecontroller may further be configured to disengage the disconnect clutchto decouple the engine and the transmission in response to the enginebeing stopped when a brake pressure is above a pressure threshold and avehicle speed is above a first speed threshold but below a second speedthreshold.

In yet another embodiment, a method for operating an automatic stop andstart system in a vehicle having an engine, a transmission and a launchdevice including a disconnect clutch for selectively coupling the engineand transmission is disclosed. The method may include controlling thelaunch device to disengage the disconnect clutch to decouple the engineand transmission in response to an automatic stop of the engine when abrake line pressure exceeds a pressure threshold and a vehicle speedexceeds a first speed threshold but is below a second speed threshold.The first speed threshold may be based on a vehicle idle speed and thesecond speed threshold is based on a rolling stop speed threshold limit.The method may further include controlling the launch device to maintaindisengagement of the disconnect clutch when a change in the brake linepressure is below a threshold value.

Embodiments according to the present disclosure may provide a number ofadvantages. For example, control of an impeller clutch during a rollingstop-start may provide fuel savings and reduced emissions associatedwith decreased operation of the engine and reduced engine drag duringvehicle deceleration. Reduced NVH during engine restart may improvecustomer satisfaction so that rolling stop-start operating modes may bereadily accepted and employed by customers. Disconnecting the enginefrom the transmission using an impeller clutch may reduce engine,transmission, and connecting component wear. Additionally, use of afluid launch clutch may provide axial space savings in comparison to aconventional torque converter. The above advantages and other advantagesand features will be readily apparent from the following detaileddescription of the preferred embodiments when taken in combination withthe accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic representation of a vehicle powertrain systemaccording to an embodiment of the present disclosure;

FIG. 2 is a schematic representation of a fluid launch device having animpeller disconnect clutch according to one or more embodiments of thepresent disclosure; and

FIG. 3 is a flowchart illustrating a method for operating an enginestart/stop system utilizing a fluid launch device with an impellerdisconnect clutch according to a representative embodiment of thepresent disclosure.

DETAILED DESCRIPTION

Embodiments of the present disclosure are described herein. It is to beunderstood, however, that the disclosed embodiments are merely examplesand other embodiments can take various and alternative forms. Thefigures are not necessarily to scale; some features could be exaggeratedor minimized to show details of particular components. Therefore,specific structural and functional details disclosed herein are not tobe interpreted as limiting, but merely as a representative basis forteaching one skilled in the art to variously employ the embodiments. Asthose of ordinary skill in the art will understand, various featuresillustrated and described with reference to any one of the figures canbe combined with features illustrated in one or more other figures toproduce embodiments that are not explicitly illustrated or described.The combinations of features illustrated provide representativeembodiments for typical applications. Various combinations andmodifications of the features consistent with the teachings of thisdisclosure, however, could be desired for particular applications orimplementations.

Vehicles equipped with start/stop systems are powered by conventionalinternal combustion engines. A controller may initiate an automatic stopor start of the engine under certain operating conditions. For example,the start/stop system may automatically stop the engine when the vehicleis stopped or decelerating and the engine is not required for propulsionor other purposes. At a later time, the start/stop system may restartthe engine when required for propulsion or other purposes, e.g., whenthe brake pedal is released and/or the accelerator pedal is actuated. Bydisabling the engine when possible, overall fuel consumption is reduced.

One type of start/stop system is known as a rolling start/stop (“RSS”).A RSS system involves stopping the internal combustion engine when thevehicle is moving. In particular, a vehicle equipped with RSS technologyis configured to shut down the engine below a defined vehicle speedthreshold as the driver applies the brakes. The defined vehicle speedthreshold at which a RSS strategy can be implemented is based on thevehicle application. One of the challenges with RSS technology invehicles equipped with automatic transmissions is that during a RSSevent the engine stays rigidly connected to the drivetrain through useof a conventional torque converter. When the engine is automaticallystopped after application of the brake or while a vehicle is idling at atraffic light, a conventional torque converter is still pumping fluidbut no useful work is being done, reducing the fuel economy.Accordingly, embodiments of the present disclosure provide systems andmethods for implementing a RSS strategy in a vehicle with an automatictransmission that improve fuel economy without drivability impactthrough use of a fluid launch device with an impeller disconnect clutch.

Referring to FIG. 1, a schematic diagram of a vehicle powertrain 10 isillustrated according to an embodiment of the present disclosure. FIG. 1illustrates representative relationships among the components. Physicalplacement and orientation of the components within the vehicle may vary.The vehicle powertrain 10 includes a vehicle system controller (VSC) 12that has appropriate logic/controls for implementing an enginestart/stop system. The VSC 12 receives signals from an accelerator pedalposition sensor (APPS) 14 and a brake pedal position sensor (BPPS) 16 todetermine vehicle acceleration and deceleration demands.

The vehicle powertrain 10 includes an engine 22 that drives transmission24. An engine control unit (ECU) 18 is configured to control the engine22 and a transmission control unit (TCU) 20 is configured to controloperation of the transmission 24 and a fluid launch clutch 26. The VSC12 transfers data between the TCU 20 and ECU 18 and is also incommunication with various vehicle sensors. Engine 22 generatesmechanical power by converting stored chemical energy in a fuel source.Transmission 24 adapts the speed and torque of the mechanical powerproduced by the engine 22 to suit the current needs of the vehicle.Mechanical power from transmission 24 is routed to wheels 28 bydifferential 30.

The transmission, or gearbox, 24 may include gear sets (not shown) thatare selectively placed in different gear ratios by selective engagementof friction elements such as clutches and brakes (not shown) toestablish the desired drive ratios. The transmission 24 is automaticallyshifted from one ratio to another based on various vehicle and ambientoperating conditions by an associated controller, such as TCU 20. Thetransmission, or gearbox, 24 then provides powertrain output torque tooutput shaft 44.

Fluid launch clutch 26 transmits power and torque from engine crankshaft40 to transmission input shaft 42 of transmission 24. The fluid launchclutch 26 may include an impeller clutch 32, impeller 34, and turbine36. The impeller clutch 32 of the fluid launch clutch 26 may becontrolled to selectively couple the engine 22 with the impeller 34 andtransmission 24. A bypass clutch 38 may also be provided that, whenengaged, frictionally or mechanically couples the impeller 34 to theturbine 36 of the fluid launch clutch 26. The bypass or lock-up clutch38 for the fluid launch clutch 26 may be selectively engaged to create amechanical connection between the impeller side and the turbine side ofthe fluid launch clutch 26 for direct torque transfer from enginecrankshaft 40 to the transmission input shaft 42.

In general, the control system for the vehicle powertrain 10 may includeany number of controllers, such as VSC 12, ECU 18 and TCU 20, and may beintegrated into a single controller, or have various modules. Some orall of the controllers may be connected by a controller area network(CAN) or other system. The control system may be configured to controloperation of various components of the transmission 24, fluid launchclutch 26 and engine 22 under any of a number different conditions,including an engine start/stop system. The control system, including VSC12, ECU 18 and TCU 20, may implement an engine start/stop system by, atappropriate times, stopping engine 22 by halting fuel and restarting theengine 22 when propulsion is required.

The control system controls various actuators in response to signalsfrom various sensors to control functions such as starting/stoppingengine 22, operating impeller clutch 32 of fluid launch clutch 26 toselectively decouple impeller 34 from the driveline, selecting orscheduling transmission shifts, etc. The control system may include amicroprocessor or central processing unit (CPU) in communication withvarious types of computer readable storage devices or media. Computerreadable storage devices or media may include volatile and nonvolatilestorage in read-only memory (ROM), random-access memory (RAM), andkeep-alive memory (KAM), for example. KAM is a persistent ornon-volatile memory that may be used to store various operatingvariables while the CPU is powered down. Computer-readable storagedevices or media may be implemented using any of a number of knownmemory devices such as PROMs (programmable read-only memory), EPROMs(electrically PROM), EEPROMs (electrically erasable PROM), flash memory,or any other electric, magnetic, optical, or combination memory devicescapable of storing data, some of which represent executableinstructions, used by the controller in controlling the engine orvehicle.

FIG. 2 schematically illustrates a fluid launch clutch 200 according toone or more embodiments of the present disclosure. The fluid launchclutch is similar to a torque converter without a stator and thereforelacks the ability to multiply engine torque. Because the fluid launchclutch does not have a stator, it provides axial space savings incomparison to a conventional torque converter. The fluid coupling isdesigned to be engaged only for low speed maneuvers, such as creep andparking lots, and initially during a launch event. The fluid launchclutch has the same ability for impeller disconnect at idle and allowingfor impeller slip during a launch for faster time to boost. The methodof controlling the impeller and lock-up clutches can be controlled toachieve desired vehicle performance. This is accomplished by controllingthe amount of torque on these clutches. Reduced axial space requirementsof the fluid launch clutch allows for the potential to add additionalgear sets in the transmission, which further increases the ratio spread.With more gear ratio spread, there is less need for torquemultiplication, making the fluid launch clutch an improvement forvehicle powertrains.

Fluid launch clutch 200 provides two parallel power flow paths fromengine crankshaft 202 to transmission input shaft 204. A hydrodynamicpower flow path includes impeller clutch 206, impeller 208, and turbine210. Impeller 208 is selectively coupled to engine crankshaft 202 byimpeller clutch 206. An impeller clutch 206 is an actively controlledfriction clutch that selectively couples an impeller 208 of the fluidlaunch clutch 200 to the engine crankshaft 202. The impeller clutch 206allows for the engine and the transmission to be decoupled from thedriveline during certain vehicle events, such as during a RSS event.Turbine 210 is fixedly coupled to transmission input shaft 204. Enginecrankshaft 202 is selectively coupled to transmission input shaft 204 bybypass or lock-up clutch 212 providing a second power flow path.

During a launch condition, pressure within the fluid launch clutch 200is increased to engage impeller clutch 206, while the bypass clutch 212remains disengaged. The impeller clutch 206 is engaged to connectimpeller 208 to the vehicle engine so that torque received from enginecrankshaft 202 is output to transmission input shaft 204 through turbine210. Impeller 208 directs fluid into the turbine 210 to transmissioninput shaft 204 to propel the vehicle. After initial launch, pressurewithin the fluid launch clutch 200 is increased to a sufficient level toengage bypass clutch 212. Engagement of bypass or lock-up clutch 212bypasses the fluid circuit so that torque is transmitted directly fromthe engine crankshaft 202 to the transmission input shaft 204. Whereas,during a vehicle stop or RSS event, the impeller clutch 206 and thebypass clutch 212 can be disengaged so the impeller does not rotate andfuel economy can be improved.

Both impeller clutch 206 and bypass clutch 212 are actively controlledfriction clutches with torque capacities that respond to changes influid pressure in hydraulic circuits. The hydraulic circuits may bededicated circuits whose only function is to control the clutch.Alternatively, the hydraulic circuits may also be used for otherfunctions such as supplying fluid to the fluid launch clutch torus.

The control logic or functions described above may be represented byflow charts or similar diagrams in one or more figures. These figuresprovide representative control strategies and/or logic that may beimplemented using one or more processing strategies such asevent-driven, interrupt-driven, multi-tasking, multi-threading, and thelike. As such, various steps or functions illustrated may be performedin the sequence illustrated, in parallel, or in some cases omitted.Although not always explicitly illustrated, one of ordinary skill in theart will recognize that one or more of the illustrated steps orfunctions may be repeatedly performed depending upon the particularprocessing strategy being used.

Referring to FIG. 3, a control algorithm 300 illustrating a method foroperating an engine start/stop system utilizing a fluid launch clutchwith an impeller disconnect clutch is described. The control algorithmbegins at block 302. A set of entry conditions are then evaluated atblocks 304-312. At decision block 306, a first brake pressure P_(brk1)is compared with a first pressure threshold P_(thd1). The first pressurethreshold P_(thd1) is a value associated with a driver's intent to begina rolling auto stop event. If the first brake pressure P_(brk1) is lessthan the first pressure threshold P_(thd1) at decision block 306, thenthe control algorithm returns to the beginning to evaluate the entryconditions again at block 304. If the first brake pressure P_(brk1) isgreater than or equal to the first pressure threshold P_(thd1) atdecision block 306, then the control algorithm compares a vehicle speedV_(sp) with a low-speed threshold V_(sp) _(_) _(low) at block 308. Thelow-speed threshold V_(sp) _(_) _(low) may be a value associated withvehicle idle speed.

If the vehicle speed V_(sp) is less than the low-speed threshold V_(sp)_(_) _(low) at decision block 308, then the control algorithm returns tothe beginning to evaluate the entry conditions again at block 304. Ifthe vehicle speed V_(sp) is greater than the low-speed threshold V_(sp)_(_) _(low) at decision block 308, then the control algorithm comparesthe vehicle speed V_(sp) to a high-speed threshold V_(sp) _(_) _(high)at block 310. The high-speed threshold V_(sp) _(_) _(high) is a valueassociated with a maximum vehicle speed at which a rolling start/stopmode can be executed. If the vehicle speed V_(sp) is greater than thehigh-speed threshold V_(sp) _(_) _(high) at block 310, then the controlalgorithm returns to the beginning to evaluate the entry conditionsagain at block 304. If the vehicle speed V_(sp) is less than thehigh-speed threshold V_(sp) _(_) _(high) at block 310, then the controlalgorithm evaluates other vehicle conditions to determine whether theengine can be shutdown. Other vehicle conditions may include the batterystate of charge, loads on the electrical system, catalyst temperature,etc.

If other vehicle conditions indicate that the engine should not bestopped at this time, then the control algorithm returns to thebeginning to evaluate the entry conditions again at block 304. Whereas,if at decision block 312 it is determined after evaluating other vehicleconditions that the engine can be shutdown, then a rolling start/stop(“RSS”) strategy is implemented at block 314. During implementation ofRSS mode, the engine is automatically shut down and the impeller clutchof the fluid launch clutch is commanded to disengage to decouple theimpeller and the engine (i.e., driveline is opened). Then the controlalgorithm determines a current brake pedal pressure P_(brk2) at block316 and then calculates a difference ΔP_(brk) between the current brakepedal pressure P_(brk2) and the first brake pedal pressure P_(brk1) atblock 318.

The change or difference ΔP_(brk) between the current brake pedalpressure P_(brk2) and the first brake pedal pressure P_(brk1) is thencompared with a corresponding pressure threshold P_(thd2) at decisionblock 320. The corresponding pressure threshold P_(thd2) is a valueassociated with a driver's intent to exit the rolling auto stop event.If the change in brake line pressure ΔP_(brk) exceeds the correspondingpressure threshold P_(thd2), then this indicates a driver's intent toexit the rolling auto stop event and the control algorithm then exitsthe RSS strategy at block 324. The engine is then restarted and thedriveline is closed i.e., the impeller clutch is engaged to couple theimpeller and engine. If the change in brake line pressure ΔP_(brk) isbelow the corresponding pressure threshold P_(thd2) at block 320, thenthis indicates a driver's intent to continue the rolling auto stop eventand the control algorithm then evaluates at decision block 322 whetherother vehicle conditions require the engine to be restarted.

Other vehicle conditions may include the battery state of charge, loadson the electrical system, catalyst temperature, etc. For example, thecontroller may determine whether a vehicle power demand exceeds acurrently available electrical power, where the vehicle power demand isbased on an amount of electrical energy required to power vehicleaccessory loads and subsystems. If this is the case, then the engine mayneed to be restarted to meet the vehicle power demand. Likewise, thecontroller may determine that the engine should be restarted in responseto a battery voltage corresponding to a threshold limit.

If other vehicle conditions require the engine to be restarted atdecision block 322, then the control algorithm exits the RSS strategy,restarts the engine and the impeller clutch is engaged to close thedriveline to couple the fluid launch clutch impeller with the engine atblock 324. If other vehicle conditions do not require the engine to berestarted at decision block 322, then the control algorithm returns toblock 316 to keep evaluating the current brake pedal pressure P_(brk2)and any subsequent change in the brake line pressure ΔP_(brk). Thecontrol strategy ends at block 326.

While exemplary embodiments are described above, it is not intended thatthese embodiments describe all possible forms of the invention. Rather,the words used in the specification are words of description rather thanlimitation, and it is understood that various changes may be madewithout departing from the spirit and scope of the invention.Additionally, the features of various implementing embodiments may becombined to form further embodiments of the invention.

What is claimed is:
 1. A stop-start system for a vehicle having anengine, a transmission and a launch device including a disconnectclutch, comprising: a controller configured to, in response to an enginebeing alternately stopped and restarted during a rolling auto stopevent, control the disconnect clutch of the launch device to selectivelycouple the engine and the transmission.
 2. The stop-start system ofclaim 1, wherein the controller is further configured to disengage thedisconnect clutch to decouple the engine and the transmission inresponse to the engine being stopped when a brake pressure is above apressure threshold and a vehicle speed is above a first speed thresholdbut below a second speed threshold.
 3. The stop-start system of claim 2,wherein the first speed threshold is based on a speed associated withvehicle idle conditions and the second speed threshold is based on aspeed associated with a rolling stop threshold limit.
 4. The stop-startsystem of claim 2, wherein the controller is further configured tomaintain disengagement of the disconnect clutch when a change in thebrake pressure is below a corresponding threshold value.
 5. Thestop-start system of claim 1, wherein the controller is furtherconfigured to engage the disconnect clutch to couple the engine and thetransmission in response to the engine being restarted when a differencebetween a first brake pressure and a second brake pressure exceeds athreshold value.
 6. A method for operating an automatic stop and startsystem in a vehicle having an engine, a transmission and a launch deviceincluding a disconnect clutch for selectively coupling the engine andtransmission, comprising: controlling the launch device to disengage thedisconnect clutch to decouple the engine and transmission in response toan automatic stop of the engine when a brake line pressure exceeds apressure threshold and a vehicle speed exceeds a first speed thresholdbut is below a second speed threshold.
 7. The method of claim 6, whereinthe first speed threshold is based on a vehicle idle speed and thesecond speed threshold is based on a rolling stop speed threshold limit.8. The method of claim 6, further comprising: controlling the launchdevice to maintain disengagement of the disconnect clutch when a changein the brake line pressure is below a threshold value.
 9. The method ofclaim 8, further comprising: controlling the launch device to engage thedisconnect clutch to couple the engine and the transmission in responseto an automatic restart of the engine when the change in the brake linepressure exceeds the threshold value.
 10. The method of claim 6, furthercomprising: controlling the launch device to engage the disconnectclutch to couple the engine and the transmission in response to anautomatic restart of the engine when a vehicle power demand exceeds acurrently available electrical power.
 11. The method of claim 10,wherein the vehicle power demand is determined from an amount ofelectrical energy required to power vehicle accessory loads.
 12. Themethod of claim 6, further comprising: controlling the launch device toengage the disconnect clutch to couple the engine and the transmissionin response to an automatic restart of the engine when a battery voltagereaches a threshold limit.
 13. The method of claim 6, furthercomprising: controlling the launch device to engage the disconnectclutch to couple the engine and the transmission in response to anautomatic restart of the engine when an accelerator pedal is actuated.